This invention relates generally to composite shell structures, such as antenna reflectors, and their methods of fabrication and, more particularly, to laminated composite shell structures that are fabricated from multiple layers of overlapping strips of composite material.
Antenna reflectors deployed on orbiting spacecraft are subjected to wide temperature variations due to the alternate extremes of direct sunlight and total darkness encountered in orbit. An antenna reflector must be able to withstand the thermal stresses induced by these wide temperature variations without undergoing large distortions in shape. Distortions in the high-precision three-dimensional surface contour of an antenna reflector cause a loss in signal strength and possible loss of information, particularly for large reflectors or high frequency signals. An antenna reflector intended for use in space, therefore, should be rigid, strong and lightweight and have good thermoelastic properties.
One type of shell construction that is rigid, strong and lightweight is a laminated composite shell structure fabricated from multiple layers of overlapping strips of composite material, such as unidirectional graphite fiber reinforced epoxy tapes. The mechanical properties of a laminated composite shell structure, such as strength, stiffness and thermoelasticity, are largely determined by the type of composite material and the relative angular orientation of the fibers in the multiple layers. Strength and stiffness are maximized by orienting the fibers in the multiple layers at different angles, such as 0.degree. and 90.degree. , or 0.degree. and .+-.60.degree.. Thermoelasticity is minimized by using a composite material with a low coefficient of thermal expansion and constructing the shell structure such that any thermal expansion or contraction is quasi-isotropic. Quasi-isotropic expansion and contraction is provided by maintaining the relative angular orientation of the fibers in the multiple layers at their selected angles within a small tolerance throughout the shell structure.
If a shell structure conforming to a three-dimensional doubly-curved surface is relatively flat, the angular orientation of the fibers in the multiple layers remains the same within an acceptable tolerance throughout the shell structure. However, if a shell structure conforming to a doubly-curved surface has some curvature, large deviations result in the angular orientation of the fibers in the multiple layers. For example, if two layers of constant width strips are oriented at angles of 0.degree. and 90.degree. at the vertex of a parabolic antenna reflector having an F/D (focal length to diameter ratio) ratio of 0.25, the strips will intersect at angles from 70.degree. to 110.degree. at different locations along the periphery of the reflector. This is an excessive deviation from the desired 90.degree. orientation. Accordingly, there has been a need for a laminated composite shell construction that does not suffer from these large deviations in fiber orientation. The present invention is directed to this end.